At its height, police were "very concerned about the amount of people who were suffering from the side effects of bad synthetic drugs".
McKay said police in the Porirua area took action against a "major supplier" who they believe was involved in the distribution of synthetics in the Hutt.
He believed that could have something to do with the decrease in incidents, but could not be certain. He also pointed to education work that had been done through schools and in the media.
"Some people probably made the choice not to take it," he said.
Nationally, more than 65 people are now thought to have died from synthetic drugs, according to the Coroner.
In November last year the number of deaths linked to toxic synthetics was 45-50 since June 2017.
That figure since jumped to 60-65, with potentially more.
A spokesperson for Coronial Services said there were seven confirmed deaths from synthetic cannabis toxicity and around 55 cases that appeared to be linked.
A bill was introduced to Parliament in March which will increase penalties for makers and suppliers of synthetic drugs while giving police more discretion to deal with users.
The Misuse of Drugs Amendment Bill reclassifies AMB Fubinaca and 5F-ABD as Class A, and creates a new classification, Class C1, to give police greater search and seizure powers for other new and emerging drugs.